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The vast majority of losses to property and lives caused by an earthquake involve manmade structures and the people inside. It is often said, “Earthquakes don’t kill people, buildings do.” Because the Northridge earthquake struck directly in an urbanized environment, effects on buildings and highway structures were strikingly clear. All NEHRP agencies undertook studies to understand the extent and causes of failures of buildings and freeway structures. The USGS performed detailed analyses of instrumented buildings, studies of aftershocks recorded in heavily damaged structures, and interpretations of the large amount of data on building damage.
The USGS studied three significant buildings that were tested by strong shaking during the Northridge earthquake. The University of Southern California (USC) Hospital building in Los Angeles, about 35 kilometers from the fault, is a base-isolated structure. The Olive View Hospital (OVH) building in Sylmar, 7.3 kilometers from the fault, is a conventionally designed building that replaced the structure that was severely damaged in the 1971 San Fernando earthquake and later razed. Both buildings contained large arrays of instruments placed to assess accelerations and displacements throughout the buildings and to compare with the shaking of the nearby ground. The study also assessed the effects of the Whittier Narrows earthquake of 1987 on the OVH building for comparisons with the Northridge event. The Holiday Inn in Van Nuys was constructed in 1966, and experienced the effects of the 1971 San Fernando and 1994 Northridge quakes. This building suffered minor damage in 1971, but sustained major structural damage in 1994.
The USC Hospital is an 8-story building with many isolators that act like shock absorbers to damp out strong ground shaking. The building contains 149 isolators that support a steel superstructure on continuous concrete spread footings. Diagonally braced perimeter frames, supported by 68 isolators, were designed to carry the lateral loads and the internal columns that carry the vertical load were supported by 81 isolators. The building, within 15 kilometers of the Newport-Inglewood fault zone, was designed for a maximum relative isolator displacement of 26 centimeters, and to meet the seismic standards of the 1988 and 1994 Uniform Building Code (UBC).
The motions recorded at the top of the isolators and at the roof were smaller than those recorded below the isolators and in the free field (a ground site away from and free of the influences of the building). These findings provide clear evidence that the isolators were effective in dissipating the vibrational energy traveling from the free field to the building. In general, the shapes of the spectra of recorded components of motions are well enveloped by the building code spectrum, except for some high frequency (>1 hertz) bands for which the code spectrum is exceeded. Accelerations at various levels of the building, and the amplitude spectra and relative displacements of the isolators confirm excellent performance throughout the building. Drift ratios (displacements within a building story at the top relative to the base) reached only 10% of allowable values explaining why there was little damage to the structure or its contents. However, the ground motions at this site were only moderately strong. USGS studies showed that displacements near the seismogenic fault of the Northridge quake would exceed the designed displacement range for the isolators. Therefore, the performance of the isolators could be a problem at the USC Hospital should shaking be stronger than that of the Northridge earthquake.
|Base-isolated buildings are a relatively recent addition to U.S. earthquake-resistant design strategies. In base-isolated structures, the building is insulated from strong ground motion by energy-absorbing systems between the building and its foundation. Such buildings are still uncommon because the design concept had not been tested by severe earthquakes prior to the Northridge event.|
The structural responses derived from strong-motion recordings in buildings actually shaken by earthquakes can be compared to the theoretical responses that were used in designing the structures. These comparisons may verify the calculations and the stability of the design, or they may point to problems that need to be corrected in future construction. Performance of the USC Hospital building showed that design improvements are needed for the type of base-isolation system used in the building.
|Prior to the Northridge earthquake, structural engineers believed that modern buildings constructed with specially designed steel frames could resist very intense ground motions with only limited structural damage. However, during the earthquake, the steel framing of more than 100 such structures experienced fractures. The fractures initiated at the welded connections between the beams and columns of the frames. These fractures were initially attributed to poor quality workmanship in the welding of the connections. However, a review of historical test data and tests performed immediately following the earthquake demonstrated that connections with standard quality workmanship were vulnerable to the fractures. Many of the damaged buildings were at sites that experienced ground motions approximating those adopted by the building code as a basis for design. Consequently, the adequacy of the building code provisions was immediately questioned, and the International Conference of Building Officials (ICBO) adopted an emergency code change in October 1994. The ICBO action prompted the creation of a joint venture among the Structural Engineers Association of California (SEAOC), the Applied Technology Council (ATC), and the California Universities for Research in Earthquake Engineering (CUREE). The joint venture, simply named SAC, initiated a program to resolve the issues related to the steel-frame damage, and to publish advisories and guidelines. SAC published several interim documents during 1995 to assist engineers and building officials in design and inspection activities while reliable new building codes were being developed for steel-frame buildings. This effort was funded by the Federal Emergency Management Agency (FEMA).|
The OVH building was designed in 1976 to withstand increased levels of seismic forces based on the disastrous fate of its predecessor. The structural system for resisting lateral forces is a mixed design of concrete and steel shear walls. The foundation consists of spread footings and concrete slab-on-grade for the ground floor. The ground floor and second floor are typically built of concrete shear walls 25 centimeters thick that extend along several column lines. At the third level, the plan of the building changes to a cross shape, making a 4-story tower with steel shear walls surrounding the perimeter.
USGS engineers examined the building’s performance by using data from both the Northridge and Whittier Narrows earthquakes for comparisons. The building was designed for two levels of performance. The first level, 0.52g, represented accelerations at which the building would not be badly damaged. The second level, 0.69g, represented accelerations at which the building would survive, perhaps with major damage but without catastrophic failure.
Data from sensors in the OVH building show that the building escaped the impact of long-period (>1 second) pulses generated by the Northridge quake. The OVH records indicated that very large peak accelerations at the ground level (0.91g) and the roof level (2.31g) were accommodated by the building without structural damage. Analyses of the data indicate that the structure was in resonance at frequencies between 2.5 and 3.3 hertz. These frequencies are also within the site-response frequencies of 2-3 hertz, calculated from examining the geological materials upon which the structure was built. The effective structural frequencies derived from the Northridge and Whittier earthquakes data are different and exhibit variations attributable to nonlinear effects. One effect is soil-structure interaction which was seen to be more pronounced during the Northridge event. The building possibly experienced rocking at 2.5 hertz in the north-south direction during that event, and there is the possibility that radiation damping (wherein the building dissipates energy into the surrounding soil) contributed to reducing that response. Damping ratios for the building were 10%-15% (north-south) and 5%-10% (east-west) for the Northridge effects, and 1%-4% (north-south) and 5%-8% (east west) for the Whittier effects. These nonlinear effects that tended to reduce the shaking of the building during large ground motions are consistent with the different damping ratios observed during the Northridge and Whittier Narrows events. There was also nonlinear behavior due to minor structural damage during the Northridge earthquake. It is also likely that the cruciform wings responded with a different frequency than that of the overall building.
The Olive View Hospital building was conceived and designed as a very strong and stiff structure, particularly in response to the disastrous performance of the original OVH building during the 1971 San Fernando earthquake. However, the resulting design placed the fundamental frequency of the building within the frequency range of the site (2-3 hertz), thereby producing conditions for resonance. This case study indicates that determining the site frequencies needs to be emphasized in developing design response spectra. Despite the site resonances, the performance of the OVH building was considered to be a great success during the Northridge event. The hospital sustained no structural damage under very strong shaking (greater than 2g) at the roof level.
The stiff design of the Olive View Hospital performed very well during the Northridge earthquake, probably saving many lives in a region of very intense shaking. Although the structure did not sustain damage, the hospital had to be evacuated because of broken water pipes and other secondary damage. These elements of the interior design need to be improved in anticipation of future earthquakes.
The Holiday Inn in Van Nuys is a 7-story, reinforced concrete building built in 1966. The earthquake resistance system consists of frames around the perimeter, and a foundation of concrete pilings. The building was instrumented with three accelerographs prior to the 1971 San Fernando earthquake and, thereafter, with 16 accelerometers that recorded the Northridge quake. The building is located approximately the same distance from the seismogenic faults of both earthquakes (see p. 18). During the San Fernando earthquake, the building suffered minor damage that was mostly nonstructural. During the Northridge event, however, the building suffered extensive structural damage. USGS scientists used the records from both earthquakes to determine reasons for the difference in the amount of damage.
The peak ground-level accelerations for the San Fernando earthquake were 0.25g, 0.14g, and 0.17g in north-south (transverse), east-west (longitudinal), and vertical directions, respectively. The corresponding values for the Northridge quake were 0.42g, 0.44g, and 0.28g. The peak roof displacements relative to the ground for the San Fernando quake were 15 centimeters in the north-south direction and 8 centimeters in the east-west direction. For the Northridge quake, the peak north-south displacements were 17 centimeters for the east end of the building, and 23 centimeters for the west end. The peak east-west displacement of the roof with respect to the ground was 23 centimeters. The large difference between the east- and west-end displacements indicated a significant amount of torsion in the building. Using data from roof sensors, USGS engineers determined that the building was rotating with respect to a center near its east end during the Northridge event. The building lacked sufficient instruments during the San Fernando event to record comparative torsional modes.
USGS engineers analyzed response spectra for the building for the San Fernando and Northridge earthquakes. The analysis showed that the building responded with much larger amplitudes to the Northridge quake than to the San Fernando quake, and that the response spectra for the Northridge event exceeded the Uniform Building Code (UBC) response spectra. Ground-to-roof transfer functions showed that the north-south vibrations of the building were coupled with torsion in both earthquakes. The dominant frequency of this mode was near 0.7 hertz. The east-west vibrations had dominant frequencies of 0.9 hertz during the San Fernando earthquake and 0.5 hertz during the Northridge event. The Northridge frequencies are those after failure of the fourth-floor columns.
Ground-motion data to support the investigations described on pages 48-53 were obtained in part from the California Division of Mines and Geology (CDMG) strong motion instrumentation program (see p. 24).
The Holiday Inn in Van Nuys was severely damaged during the Northridge earthquake, after suffering only minor damage in the 1971 San Fernando earthquake. Reasons for the different responses were essentially due to the stronger shaking at this site during the Northridge quake. The building was subject to significant torsion during the Northridge event, and the spectral response of the building exceeded that of both the San Fernando earthquake and the Uniform Building Code.
The collapsed I-5/SH-14 interchange (see cover) was one of the most spectacular and costliest damage sites resulting from the earthquake. To determine what happened at the site, USGS scientists deployed instruments on the standing sections of the interchange bridges and on the surrounding ground at the bases of pier supports. The instruments were used to obtain aftershock records for (1) determining the dominant vibrational modes in which the bridges responded to shaking, and (2) estimating the main-shock motions responsible for the collapse.
Using accelerations and velocities from the four largest aftershocks, USGS engineers calculated ground-to-deck transfer functions. These functions defined the transfer of vibration from the ground to the bridge deck in the horizontal and vertical directions, and longitudinally along the bridge spans. Different methods for calculating the functions gave consistent estimates of dominant frequencies of the deck—0.4, 0.7, and 3.2 hertz in the horizontal, longitudinal, and vertical directions, respectively.
Because there were no strong-motion instruments at the bridge to record the shaking, engineers used innovative methods to estimate the main-shock motions that caused the collapse. One method used relations between the main shock and aftershocks determined for three nearby strong-motion stations. Aftershock motions from the bridge structures were then scaled by these relations, giving consistent estimates of ground motions at the bridge from the main shock. The response spectra showed very large amplitudes, particularly for the longitudinal direction, for periods less than 1 second. The peak spectral amplitudes calculated for the main shock were three to four times the design spectra in this period range.
The methods used here were an attempt to demonstrate using aftershock recordings to estimate main-shock motions. Much more detailed analysis of connectors, local geology, and other factors would be needed to determine, for example, why the collapse occurred on the lower rather than the higher sections of the structure.
USGS engineers placed instruments throughout a section of the freeway bridge adjacent to the section that collapsed during the earthquake. Using aftershock recordings and data from nearby stations that recorded the main shock, they were able to relate the collapse to accelerations that greatly exceeded those for which the bridge was designed. Accelerations calculated for the base of the bridge (1.19g and 1.02g) translated into an acceleration of 1.88g on the deck. The 1.88g value represents conditions of the failed structure, and not those of the original continuous structure.
USGS scientists studied more than 250 ground-motion records from major networks operated by the USGS, the California Division of Mines and Geology (CDMG), and the University of Southern California (USC), and smaller networks such as one operated by the Los Angeles Department of Water and Power. In general, peak accelerations exceeded those predicted by attenuation relations for California. At several locations, horizontal peaks were close to or exceeded 1g, and at one station, the peak vertical acceleration exceeded 1g. The largest horizontal peak acceleration of a free-field site was 1.78g recorded in Tarzana (see p.29).
The ground-motion data indicated a general trend of higher peak accelerations from the Northridge earthquake than for those of other California earthquakes of similar magnitude. This trend may be attributable to the thrust mechanism and the effects of directivity (see p.12). Ground motions both near and far from the epicenter contained consistent high-energy pulses of relatively long duration, a cause for concern about the vulnerability of mid-rise to highrise steel structures designed for lesser motions. Additionally, there were significant site effects contributing to the overall picture of unexpected ground motions.
The Myth of Unusual Vertical Motions
The strong-motion records show that long-duration pulses contributed to the large accelerations that damaged numerous mid-rise and highrise buildings. Long-duration pulses produced large ground velocities and displacements. Consequently, significant percentages of wave energy were transmitted to structures within the duration of the pulses, which commonly were within the 1-5 second period common to most of the buildings. Ground-velocity records analyzed for stations 10, 16, and 20 kilometers from the epicenter indicate that buildings need higher strength and larger ductility (flexibility of a structure) to accommodate the velocities without damage.
The shapes of the response spectra of motions at many sites exceeded the Uniform Building Code (UBC) spectra beyond T>0.5 seconds for “soil-like” sites (called S2 in the UBC). Many motion spectra from “rock-like” (S1) sites exceed the UBC spectra in the short-period range. The averages of the normalized spectra are enveloped reasonably well by the UBC design-response spectra for either S1 or S2 sites. The standard deviation above the average is significant, however, for S1 sites. Therefore, a spectral peak of 3.0 (compared to the current value of 2.5) for the 0.1-0.5 second range of building response should be used for S1 sites, and similar increases are proposed for S2 sites. Sites that experienced unusually high accelerations, such as in Newhall and Tarzana, bear further consideration as to special designs to accommodate such accelerations.
The California Office of Emergency Services (OES) compiled a detailed data set on building damage throughout southern California (see p. 8). The data set identifies the types, geographic distribution, and damage, if any, of buildings that were inspected and “tagged” throughout Los Angeles County. The data set is separated into 12 building categories differentiated by date and type of construction. USGS scientists used the data set to map shaking intensity in the county and to develop a method for predicting future damage and loss from earthquakes.
Scientists selected seven building categories to give the most complete areal coverage for the analysis. Five of these categories were wood-frame residential structures comprising pre- and post-1940 single-family, pre- and post-1940 2- to 4-family, and post-1940 multifamily dwellings. The other two categories were 1940-76 and post-1976 masonry structures. The data were aggregated by 1990 Census tracts in order to give the highest resolution to the coverage. In the San Fernando Valley, Census tracts cover about 1.5 square kilometers on the average, and in downtown Los Angeles, they cover about 1 square kilometer. The most densely populated Census tracts contain more than 2,000 residential structures.
USGS scientists developed a mathematical relation that estimates shaking intensity by comparing “red-tagged” (no occupancy allowed) buildings in the post-1940, single-family-dwelling category to exposed buildings in a Census tract. The estimator uses a damage matrix, derived from data from the 1989 Loma Prieta earthquake in northern California, that predicts the ratio of “red-tagged” to exposed buildings for Modified Mercalli Intensities (MMI) in the range V to X (see p. 9). The relation then is modified using damage matrices for other building categories and for the “yellow-tagged” (limited entry allowed) and “green-tagged” (no restrictions on entry) data. The resulting equation yields a tagging intensity that reflects the utility of the various tags for estimating different shaking intensity levels. For example, the “red tags” predict the higher MMI intensities (IX and X+) much more reliably than the lower intensities.
A new shaking-intensity estimator, called the tagging intensity, is based on building-inspection data. Tagging intensity, mapped by Census tracts in Los Angeles County, shows damage clusters in the San Fernando Valley in Northridge, Granada Hills, and Sherman Oaks. In the Los Angeles basin, there are damage clusters in and to the west-southwest of downtown Los Angeles. In general, the shaking intensity calculated by the estimator exhibits greater detail and variability than the MMI intensities delineated for the region.
The analysis showed that the post-1940 single-family dwellings constitute both the largest building category, and the strongest in terms of resistance to damage from earthquake shaking. Post-1940 multifamily dwellings proved to be more susceptible to shaking damages than the categories of masonry buildings or pre-1940 single-family and 2- to 4-family wood-frame dwellings.
The dense areal coverage of tagging data for Los Angeles County yielded the most detailed estimates of damage and shaking intensity ever obtained for an earthquake in the United States. Crosscorrelating damage in seven building categories revealed that the post-1940 multifamily dwellings were more susceptible to damage than the two masonry-structure categories or the two pre-1940 wood-frame dwelling categories. These results will strongly condition predictions for building damages and losses from future earthquakes in California.
Current Federal and State legislations contain a myriad of economic incentives for mitigating specific hazards to buildings, such as bolting a structure to its foundation. The Robert T. Stafford Disaster Relief and Emergency Assistance Act (Public Law 93-288), the Alquist-Priolo Special Study Zones Act (California Public Resources Code, 1974), and the California Seismic Hazards Mapping Act of 1990 all contain provisions for market incentives and administrative actions to promote public safety and property-loss reduction. As a result, a variety of market incentives have been implemented. The rules and market incentives associated with these acts require site-specific engineering study and review in recognized hazardous areas. However, many vulnerable locations are not well known and are distributed unequally in hazards-prone regions. For example, the Northridge earthquake occurred on a previously unknown fault, albeit in a region generally known for its earthquake hazards. This mismatch of information creates a market uncertainty that could affect a hazards-mitigation strategy. USGS work on this issue investigated risk assessment methods based on scientific data that can be applied to earthquake hazards. These methods are intended to evaluate the cost-effectiveness of earthquake-hazards mitigation.
The risk-assessment model can be applied to the problem of earthquake earthquake-hazards mitigation choices using the following steps:
(1) Establish a decision framework by defining a benefit-cost analysis for earthquake-hazards mitigation choices and measures;
(2) Develop a physical model that incorporates strong ground motions and site characteristics;
(3) Establish a conceptual model for safety incentives that incorporates the rules for making alternative safety decisions;
(4) Develop a descriptive model that integrates the spatial and temporal structure for earth- sciences and economic information;
(5) Formalize step (3) with empirical data to produce a prescriptive model that represents what should be done to maximize the benefits of mitigation; and
(6) Apply a probabilistic model to identify the optimal strategy from the feasible mitigation choices identified in (5).
A good example of applying economic modeling is evaluating the choices consumers make in preparing for earthquakes. People can purchase earthquake insurance, carry out personal or community mitigation efforts, do nothing, or combine several of these activities.
Consumers in an earthquake-prone region are faced with making choices about personal safety that involve such factors as the probabilities of earthquakes, potential damages, and appropriate mitigation activities. In addition, consumers need public information about these factors. The economic model combines the probability of a hazard, the property value at risk, and the cost of various mitigation activities.
In general, the decisions of an educated consumer are based on avoiding losses. Thus, if a consumer is classified as benefiting from a strategy that reduces losses, there is a net gain in consumer welfare, and the consumer has an incentive to purchase mitigation measures. Under these conditions, it is clear that public education and information can result in large economic savings within a hazardous region. In an ideal world, consumers would be informed by public agencies of the time, location, magnitude, and local site effects of a future earthquake, and would make rational decisions. However, scientific data about the recurrence of damaging earthquakes and the geographic vulnerability of specific sites cannot be precisely determined. We have seen that it is very difficult to predict even the location of damaging earthquakes. Therefore, occurrences of seismic hazards typically are stated in terms of spatial and temporal probabilities that can be used in decision making.
Earthquake insurance and building codes are accepted measures to reduce damages. Earthquake insurance policies have seemingly large deductibles and substantial annual premiums. For example, a representative policy will include a deductible of 10% of insured value and an annual premium of $2 per $1000 of coverage. Earthquake-related building codes generally are minimum provisions that are necessary for ensuring structural integrity during shaking. However, such codes commonly do not address the collateral effects of earthquakes (such as landsliding) that temporarily exacerbate the potential for damage. Mitigation for a specific hazard may involve major construction activity, such as foundation alteration, retrofitting internal structural support, or other expensive or extraordinary measures. Thus, for each mitigation choice there is a planned beneficial effect and a known cost to achieve the specified level of protection.
One of the real successes of earthquake preparedness in southern California is the Los Angeles City program to retrofit unreinforced masonry buildings (URM’s). Since 1982 several thousand of these structures have been repaired, some in areas that were strongly shaken by the Northridge earthquake. Although it is difficult to quantify the benefits of retrofitting, the savings in lives and property for the Northridge earthquake alone has more than justified this ordinance.
This masonry building near downtown Los Angeles was badly damaged during the Northridge earthquake. Prior retrofitting, evidenced by the lines of bolts (reddish, circular features indicated by the arrow), probably prevented more extensive collapse.
Analyses of the economic impacts of the Northridge earthquake demonstrated that pre-event mitigation decisions helped avoid significant loss of life and additional property damage. However, there will be more damaging earthquakes in the future in the region. Economic modeling provides a means for consumers to evaluate different mitigation choices and incentives. Using a refined model with more complete earth-science information, consumers can apply economically sound decisions about risk-based regulations or insurance programs containing incentives for mitigation.
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